Surface smoothing of stereolithographically formed 3-D objects

ABSTRACT

A stereolithographic method and apparatus for forming polymeric structures comprising a plurality of overlying layers, each layer formed by polymerizing a thin layer of liquid photopolymer on a prior layer. Crevices formed at the layer interfaces are filled by a stereolithographic method comprising lifting the multilayered structure from the liquid photopolymer, draining excess liquid therefrom, tilting the structure to provide an acute angle of incidence between an incident radiation beam and a side wall of the object, and applying radiation to the crevice to polymerize at least the surface of a quantity of liquid photopolymer therein. The structure may then be subjected to a separate final full cure to fully harden the structure. An exemplary use is the packaging of electronic components and the like.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a divisional of application Ser. No.09/634,239, filed Aug. 8, 2000, pending.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to stereolithography and,more specifically, to the use of stereolithography in forming multilayerstructures with vertical or near-vertical sides, such structuresincluding packages for semiconductor devices and the like. Mostparticularly, the present invention relates to forming multilayerstructures with sides of enhanced smoothness.

[0004] 2. State of the Art

[0005] In the past decade, a manufacturing technique termed“stereolithography”, also known as “layered manufacturing”, has evolvedto a degree where it is employed in many industries.

[0006] Essentially, stereolithography as conventionally practiced,involves utilizing a computer to generate a three-dimensional (3-D)mathematical simulation or model of an object to be fabricated, suchgeneration usually being effected with 3-D computer-aided design (CAD)software. The model or simulation is mathematically separated or“sliced” into a large number of relatively thin, parallel, usuallyvertically superimposed layers, each layer having defined boundaries andother features associated with the model (and thus the actual object tobe fabricated) at the level of that layer within the exterior boundariesof the object. A complete assembly or stack of all of the layers definesthe entire object, and surface resolution of the object is, in part,dependent upon the thickness of the layers.

[0007] The mathematical simulation or model is then employed to generatean actual object by building the object, layer by superimposed layer. Awide variety of approaches to stereolithography by different companieshas resulted in techniques for fabrication of objects from both metallicand nonmetallic materials. Regardless of the material employed tofabricate an object, stereolithographic techniques usually involvedisposition of a layer of unconsolidated or unfixed materialcorresponding to each layer within the object boundaries, followed byselective consolidation or fixation of the material to at least asemisolid state in those areas of a given layer corresponding toportions of the object, the consolidate or fixed material also at thattime being substantially concurrently bonded to a lower layer. Theunconsolidated material employed to build an object may be supplied inparticulate or liquid form, and the material itself may be consolidatedor fixed or a separate binder material may be employed to bond materialparticles to one another and to those of a previously formed layer. Insome instances, thin sheets of material may be superimposed to build anobject, each sheet being fixed to a next lower sheet and unwantedportions of each sheet removed, a stack of such sheets defining thecompleted object. When particulate materials are employed, resolution ofobject surfaces is highly dependent upon particle size, whereas when aliquid is employed, surface resolution is highly dependent upon theminimum surface area of the liquid which can be fixed and the minimumthickness of a layer which can be generated. Of course, in either case,resolution and accuracy of object reproduction from the CAD file is alsodependent upon the ability of the apparatus used to fix the material toprecisely track the mathematical instructions indicating solid areas andboundaries for each layer of material. Toward that end, and dependingupon the layer being fixed, various fixation approaches have beenemployed, including particle bombardment (electron beams), disposing abinder or other fixative (such as by inkjet printing techniques), orirradiation using heat or specific wavelength ranges.

[0008] An early application of stereolithography was to enable rapidfabrication of molds and prototypes of objects from CAD files. Thus,either male or female forms on which mold material might be disposedmight be rapidly generated. Prototypes of objects might be built toverify the accuracy of the CAD file defining the object and to detectany design deficiencies and possible fabrication problems before adesign was committed to large-scale production.

[0009] In more recent years, stereolithography has been employed todevelop and refine object designs in relatively inexpensive materialsand has also been used to fabricate small quantities of objects wherethe cost of conventional fabrication techniques is prohibitive, such asin the case of plastic objects conventionally formed by injectionmolding. It is also known to employ stereolithography in the customfabrication of products generally built in small quantities or where aproduct design is rendered only once. Finally, it has been appreciatedin some industries that stereolithography provides a capability tofabricate products, such as those including closed interior chambers orconvoluted passageways, which cannot be fabricated satisfactorily usingconventional manufacturing techniques.

[0010] To the inventors' knowledge, stereolithography has yet to beapplied to mass production of articles in volumes of thousands ormillions, or employed to produce, augment or enhance products includingother, pre-existing components in large quantities, where minutecomponent sizes are involved, and where extremely high resolution and ahigh degree of reproducibility of results is required. Furthermore,conventional stereolithography apparatus and methods fail to address thedifficulties of precisely locating and orienting a number ofpre-existing components for stereolithographic application of materialthereto without the use of mechanical alignment techniques or tootherwise assuring precise, repeatable placement of components.

[0011] In the electronics industry, state-of-the-art packaging ofsemiconductor dice is an extremely capital-intensive proposition. Inmany cases, discrete semiconductor dice carried on, and electricallyconnected to, leadframes are individually packaged with a filled polymermaterial in a transfer molding process. A transfer molding apparatus isextremely expensive, costing at least hundreds of thousands of dollarsin addition to the multi-hundred thousand dollar cost of the actualtransfer molding dies in which strips of leadframes bearingsemiconductor dice are disposed for encapsulation.

[0012] Encapsulative packaging of a semiconductor device already mountedon a substrate by molding and other presently used methods may be verydifficult, time-consuming and costly. In some cases, the device may bepackaged using a so-called “glob-top” material such as a silicone gel,but the package boundaries are imprecisely defined, a dam structure maybe required to contain the slumping gel material, and the seal achievedis generally nonhermetic.

SUMMARY OF THE INVENTION

[0013] The present invention includes a method of forming a preciselydimensioned structure from a photopolymer material by astereolithographic process. The structure is formed by creating one ormore layers of at least partially polymerized material adjacent apreformed electronic component or other small component with a highdegree of precision to create a wall adjacent thereto or, optionally, anencapsulative package therefor. For example, a semiconductor die may beprovided with a protective structure in the form of a layer ofdielectric material having a controlled thickness or depth over oradjacent one or more surfaces thereof. As used herein, the term“package” as employed with reference to electrical components includespartial, as well as full, covering or encapsulation of a givensemiconductor die surface with a dielectric material, and specificallyincludes fabrication of a semiconductor die configured in a so-called“chip-scale” package, wherein the package itself, including the die, isof substantially the same dimensions as, or only slightly larger than,the die itself.

[0014] The packaging method of the present invention may be applied, byway of example and not limitation, to a die mounted to a leadframe(having a die mounting paddle or in a paddle-less leads-over-chip (LOC),or in a leads-under-chip (LUC) configuration), mounted to a carriersubstrate in a chip-on-board (COB) or board-on-chip (BOC) arrangement, asemiconductor die in a so-called “flip-chip” configuration, or in otherpackaging designs, as desired.

[0015] The present invention employs computer-controlled, 3-D CADinitiated, stereolithographic techniques to apply protective andalignment structures to an electronic component such as a semiconductordie. A dielectric layer or layer segments may be formed over or adjacenta single die or substantially simultaneously over or adjacent a largenumber of dice or die locations on a semiconductor wafer or otherlarge-scale semiconductor substrate, individual dice or groups of dicethen being singulated therefrom. The package may be formed to cover thelateral surfaces as well as the upper and/or lower surfaces of asemiconductor die.

[0016] Precise mechanical alignment of singulated semiconductor dice orlarger semiconductor substrates having multiple die locations is notrequired to practice the method of the present invention, which includesthe use of machine vision to locate dice and features or othercomponents thereon or associated therewith (such as leadframes, bondwires, solder bumps, etc.) or features on a larger substrate foralignment and material disposition purposes.

[0017] In one embodiment, packaging for electronic components accordingto the invention is fabricated using precisely focused coherentelectromagnetic radiation in the form of an ultraviolet (UV) wavelengthlaser under control of a computer and responsive to input from a machinevision system such as a pattern recognition system to fix or cure aliquid material in the form of a photopolymer.

[0018] A multilayer package structure is formed by placing an object ina bath of photopolymer material to a depth forming a thin liquid layerwhich will comprise the lowermost layer of the package structure. Alaser beam of coherent radiation is controllably passed over selectedportions of the thin layer of photopolymer material for partialpolymerization thereof. The object is then lowered to a depth to form asecond thin liquid layer of photopolymer material over the at leastpartially polymerized prior layer, followed by laser exposure. A stackof at least partially polymerized layers is thus formed, comprising asmany consecutive, at least partially superimposed layers as are requiredto achieve the desired structure height.

[0019] In the structure fabrication process, small interstitialhorizontal crevices are defined at the joints between adjacent layers ofthe structure. Unpolymerized liquid photopolymer material forms ameniscus in each of the crevices. As such uncured material is typicallyrinsed from the structure after it is removed from the bath, asubsequent complete cure of the photopolymer of the structure outside ofthe bath does not fill the crevices, but leaves such crevices asunsightly, rough surface features which reduce the effective wallthickness of the structure and may also undesirably collect dust, dirtand moisture.

[0020] The present invention includes methods and apparatus forsubstantially eliminating these interlayer crevices and smoothing thejoints between the structure layers. Following the exemplary formationof a desired multilayer package structure about an object such as asemiconductor die, the die with surrounding package structure is removedfrom the photopolymer bath and excess liquid photopolymer drainedtherefrom. The object is then tilted by about 5-90 degrees from thehorizontal so as to reorient the side walls thereof to face at leastpartially upwardly, and the crevices between horizontal at leastpartially cured photopolymer layers, each containing a meniscus ofunpolymerized liquid photopolymer, are subjected to radiation of anappropriate wavelength to polymerize the liquid meniscus material andsmooth the exterior surfaces of the package structure.

[0021] It is also contemplated that the present invention has utilitywith respect to the formation of stand-alone structures and not merelystructures fabricated in association with pre-existing objects, such asthe aforementioned semiconductor dice or other electronic components.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0022]FIG. 1 is a schematic side elevation of an exemplarystereolithography apparatus of the invention suitable for use inpracticing the method of the present invention;

[0023]FIG. 1A is an enlarged cross-sectional side view of a portion ofthe support platform of a stereolithographic apparatus of the inventionforming a simple object;

[0024]FIG. 1B is an enlarged cross-sectional side view of a portion ofthe support platform of a stereolithographic apparatus of the inventionforming a package on a semiconductor die;

[0025]FIG. 2 is a schematic top elevation of a plurality of workpiecesin the form of semiconductor dice disposed on a platform of thestereolithographic apparatus of FIG. 1;

[0026]FIG. 3 is a schematic side elevation of a plurality of workpiecesin the form of semiconductor dice disposed on a platform of thestereolithographic apparatus of FIG. 1 for packaging in accordance withthe present invention;

[0027]FIG. 4 is a schematic cross-sectional side elevation of asemiconductor die undergoing a stereolithographic packaging step in amethod of the present invention;

[0028]FIG. 5 is a schematic cross-sectional side elevation of asemiconductor die undergoing a stereolithographic packaging step in amethod alternate to the method of the present invention;

[0029]FIG. 6 is a schematic cross-sectional side elevation of asemiconductor die packaged by a stereolithographic packaging methodalternative to the method of the present invention;

[0030]FIG. 7 is a schematic cross-sectional side elevation of asemiconductor die undergoing stereolithographic packaging by the methodand apparatus of the present invention;

[0031]FIG. 8 is a schematic cross-sectional side elevation of asemiconductor die in a step of a stereolithographic packaging method inaccordance with the present invention;

[0032]FIG. 9 is a schematic cross-sectional side elevation of asemiconductor die packaged by a stereolithographic packaging method inaccordance with the present invention; and

[0033]FIG. 10 is a schematic side elevation of an exemplarystereolithographic apparatus modified in accordance with the inventionand shown in a wall-smoothing operation of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0034]FIG. 1 depicts schematically various components and operation ofan exemplary stereolithography apparatus 10 modified to create miniaturemultilayer structures with side walls of perceptibly improvedsmoothness. Those of ordinary skill in the art will understand andappreciate that apparatus of other designs and manufacture may bemodified to practice the method of the present invention. The preferredbasic stereolithography apparatus which may be modified in accordancewith the present invention, as well as conventional operation of suchapparatus, are described in great detail in United States Patentsassigned to 3D Systems, Inc. of Valencia, Calif. such patents including,without limitation, U.S. Pat. Nos. 4,575,330; 4,929,402; 4,996,010;4,999,143; 5,015,424; 5,058,988; 5,059,021; 5,096,530; 5,104,592;5,123,734; 5,130,064; 5,133,987; 5,141,680; 5,143,663; 5,164,128;5,174,931; 5,174,943; 5,182,055; 5,182,056; 5,182,715; 5,184,307;5,192,469; 5,192,559; 5,209,878; 5,234,636; 5,236,637; 5,238,639;5,248,456; 5,256,340; 5,258,146; 5,267,013; 5,273,691; 5,321,622;5,344,298; 5,345,391; 5,358,673; 5,447,822; 5,481,470; 5,495,328;5,501,824; 5,554,336; 5,556,590; 5,569,349; 5,569,431; 5,571,471;5,573,722; 5,609,812; 5,609,813; 5,610,824; 5,630,981; 5,637,169;5,651,934; 5,667,820; 5,672,312; 5,676,904; 5,688,464; 5,693,144;5,695,707; 5,711,911; 5,776,409; 5,779,967; 5,814,265; 5,840,239;5,854,748; 5,855,718; and 5,855,836. The disclosure of each of theforegoing patents is hereby incorporated herein by this reference. Thestereolithographic apparatus may be modified as described in co-pendingU.S. patent application Ser. No. 09/259,142 filed Feb. 26, 1999,assigned to the assignee of the present invention and herebyincorporated herein by this reference. This earlier application relatesto the use of a “machine vision” system with suitable programming of thecomputer controlling the stereolithographic process, eliminating theneed for accurate positioning or mechanical alignment of workpieces towhich material is stereolithographically applied, and expands the use tolarge numbers of workpieces which may have differing orientation, size,thickness and surface topography.

[0035] While the workpieces employed in the practice of the preferredembodiment of the method of the invention are, by way of example only,semiconductor dice, wafers, partial wafers, other substrates ofsemiconductor material or carrier substrates bearing integrated circuitson dice or other semiconductor structures, the method and apparatus ofthe invention are applicable to fabricating other products includingworkpieces having the aforementioned variations in orientation, size,thickness and surface topography.

[0036] With reference again to FIGS. 1, 1A, and 1B, a 3-D CAD drawing ofa structure 40 to be fabricated in the form of a data file is placed inthe memory of a computer 12 controlling the operation of apparatus 10,if computer 12 is not a CAD computer in which the original object designis effected. In other words, an object design may be effected in a firstcomputer in an engineering or research facility and the data filestransferred via wide or local area network, tape, disc, CD-ROM orotherwise as known in the art to computer 12 of apparatus 10 for objectfabrication.

[0037] The data is preferably formatted in an STL (forStereoLithography) file, STL being a standardized format employed by amajority of manufacturers of stereolithography equipment. Fortunately,the format has been adopted for use in many solid-modeling CAD programs,so often translation from another internal geometric database format isunnecessary. In an STL file, the boundary surfaces of a structure 40 aredefined as a mesh of interconnected triangles.

[0038] Apparatus 10 also includes a reservoir 14 (which may comprise aremovable reservoir interchangeable with other reservoirs containingdifferent materials) of liquid material 16 to be employed in fabricatingthe intended structure 40. In the currently preferred embodiment, theliquid material 16 is a photo-curable polymer (hereinafter“photopolymer”) responsive to light in the UV wavelength range. Thesurface level 18 of the liquid material 16 is automatically maintainedat an extremely precise, constant magnitude by devices known in the artresponsive to output of sensors within apparatus 10 and preferably undercontrol of computer 12. U.S. Pat. No. 5,174,931, referenced above andpreviously incorporated herein by reference, discloses one suitablelevel control system.

[0039] A support platform or elevator 20 is shown, having an uppersurface 30 and moved by platform actuator 36. Platform 20 is preciselyvertically movable by actuator 36 via platform controller 32 in fine,repeatable increments responsive to control of computer 12, and islocated for movement 46 downward into and upward out of liquid material16 in reservoir 14. In addition, under the actuation of actuator 36,platform 20 is controllably tiltable by movement 48 to an acute angle 62with the horizontal plane (see FIG. 7). Furthermore, in a preferredembodiment, platform 20 is rotatable by movement 68 about a verticalaxis 70. The platform 20 and/or structures 40 placed on the platformcomprise a base upon which structures 40 are formed by astereolithographic process in this invention.

[0040] A laser 22 for generating a beam of light 26 in the UV wavelengthrange has associated therewith appropriate optics and galvanometers. Thelaser beam 26 is reflected by reflective apparatus 24 to shape anddefine beam 26 into beam 28, which is directed downwardly to the surface30 of platform 20 and traversed in the X-Y plane, that is to say, in ahorizontal plane, in a selected pattern under control of computer 12.Liquid photopolymer material 16 which is exposed to laser beam 28 as itis scanned in an X-Y plane is at least partially cured thereby to atleast a semisolid state.

[0041] Data from the STL files resident in the memory 34 of computer 12is manipulated to build a structure 40 one layer 50 at a time. Thestructure 40 is constructed on a base which may comprise the platform20, a pre-existing object 44 on the platform 20, or other object.Accordingly, the data mathematically representing structure 40 isdivided into subsets, each subset representing a slice or layer 50 ofstructure 40. This is effected by mathematically sectioning the 3-D CADmodel into a plurality of horizontal layers 50, a “stack” of such layersrepresenting object 40. Each slice or layer 50 may be from about 0.0001inch to about 0.0300 inch thick. The preferred range of layer thicknessis from about 0.002 inch to about 0.020 inch. A slice or layer 50 with arelatively small layer thickness 52 promotes higher resolution byenabling better reproduction of fine vertical surface features ofstructure 40. On the other hand, a structure 40 formed of layers 50having greater thickness 52 will have fewer layers; thus, it isconstructed with fewer scans of the laser beam 28 and the overallproduction rate is typically higher.

[0042] In some instances, a base support or supports 42 for a structure40 or pre-existing object 44 may also be programmed as a separate STLfile. The use of such base supports 42 is exemplified in FIGS. 1A and1B, which are enlarged views of a portion of the platform 20 on which astructure 40 is to be fabricated. In FIG. 1A, a structure 40 is to beconstructed on prior-formed base supports 42. The exemplary structure 40is depicted as formed of 4 layers 50A, 50B, 50C and 50D, each formed bya scan of a laser. In FIG. 1B, a structure in the form of a protectivepolymeric package 40 is to be formed by STL over a pre-existing object44, e.g., a semiconductor die. Base supports 42 are first fabricated onsurface 30 of the platform 20 to support and attach the die to theplatform. Then, the overlying structure 40 is formed by a plurality oflaser scans, each at a higher elevation. Such supports 42 facilitatefabrication of a structure 40 with reference to a perfectly horizontalplane above the surface 30 of platform 20. The structure 40 may beconstructed upon or adjacent to a pre-existing object 44 such as asemiconductor die, electronic substrate, or the like. The formation of abase support 42 between the pre-existing object 44 and the platformsurface 30 enables rigid and precise positioning of the pre-existingobject 44 in a desired precise orientation on the platform surface.

[0043] Where a “recoater” blade 38 is employed as described below, theinterposition of base supports 42 precludes inadvertent contact of blade38 with platform surface 30.

[0044] Before fabrication of structure 40 is initiated with apparatus10, the primary STL file for structure 40 and the file for basesupport(s) 42 (if used) are merged. It should be recognized that, whilereference has been made to a single structure 40, multiple structures 40may be concurrently fabricated on surface 30 of platform 20. In such aninstance, the STL files for the various structures 40 and supports 42,if any, are merged. Operational parameters for apparatus 10 are thenset, for example, to adjust the size (diameter, if circular) of thelaser light beam 28 used to cure material 16.

[0045] Before initiation of a first support layer for a support 42, or afirst layer 50A for a structure 40 is commenced, computer 12automatically checks and, if necessary, adjusts by means known in theart as referenced above, the surface level 18 of liquid material 16 inreservoir 14 to maintain same at an appropriate focal length for laserbeam 28. U.S. Pat. No. 5,174,931, referenced above and previouslyincorporated by reference, discloses one suitable level control system.Alternatively, the height of reflective apparatus 24 may be adjustedresponsive to a detected surface level 18 to cause the focal point oflaser beam 28 to be located precisely at the surface of liquid material16 at surface level 18 if level 18 is permitted to vary, although thisapproach is somewhat more complex.

[0046] The platform 20 may then be submerged in liquid material 16 inreservoir 14 to a depth equal to the thickness 52 of one layer or slice50 of the structure 40, and the liquid surface level 18 readjusted asrequired to accommodate liquid material 16 displaced by submergence ofplatform 20. Laser 22 is then activated so that laser beam 28 will scanliquid material 16 over surface 30 of platform 20 to at least partiallycure (e.g., at least partially polymerize) liquid material 16 atselected locations, defining the boundaries of a first layer 50 (ofstructure 40 or support 42, as the case may be) and filling in solidportions thereof.

[0047] Platform 20 is then lowered by a distance equal to the thickness52 of a layer 50, and the laser beam 28 scanned to define and fill inthe second layer 50B while simultaneously bonding the second layer tothe first. The process is then repeated, layer by layer, until structure40 is completed.

[0048] If a recoater blade 38 is employed, the process sequence issomewhat different. In this instance, the surface 30 of platform 20 islowered into liquid material 16 below surface level 18, then raisedthereabove until it is precisely one layer's thickness 52 below blade38. Blade 38 then sweeps horizontally over surface 30, or (to save time)at least over a portion thereof on which a structure 40 is to befabricated, to remove excess liquid material 16 and leave a film thereofof the precise, desired thickness 52 on surface 30. Platform 20 is thenlowered so that the surface of the film and material level 18 arecoplanar and the surface of the material 16 is still. Laser 22 is theninitiated to scan with laser beam 28 and define the first layer 50A. Theprocess is repeated, layer by layer, to define each succeeding layer 50and simultaneously bond same to the next lower layer 50 until structure40 is completed. A more detailed discussion of this sequence andapparatus for performing same is disclosed in U.S. Pat. No. 5,174,931,previously incorporated herein by reference.

[0049] As an alternative to the above approach to preparing a layer ofliquid material 16 for scanning with laser beam 28, a layer of liquidmaterial 16 may be formed on surface 30 by lowering platform 20 to floodmaterial 16 over surface 30 or over the highest completed layer 50 ofstructure 40, then raising platform 20 and horizontally traversing aso-called “meniscus” blade across the platform 20 (or just the formedportion of structure 40) one layer thickness 52 thereabove, followed byinitiation of laser 22 and scanning of beam 28 to define the next higherlayer 50.

[0050] Yet another alternative to layer preparation of liquid material16 is to merely lower platform 20 to a depth equal to that of a layer ofliquid material 16 to be scanned, and then traverse a combination floodbar and meniscus bar assembly (not shown) horizontally over platform 20(or merely over structure 40) to substantially concurrently flood liquidmaterial 16 over platform 20 and define a precise layer thickness 52 ofliquid material 16 for scanning.

[0051] All of the foregoing methods and apparatus for liquid materialflooding and layer thickness control are known in the art.

[0052] Each layer 50 of structure 40 is preferably built by firstdefining any internal and external object boundaries of that layer 50with laser beam 28, then hatching solid areas of object 40 with laserbeam 28. If a particular part of a particular layer 50 is to form aboundary of a void in the structure 40 above or below that layer 50,then the laser beam 28 is scanned in a series of closely-spaced,parallel vectors so as to develop a continuous surface or skin withimproved strength and resolution. The time it takes to form each layer50 depends upon its geometry, surface tension and viscosity of material16, thickness 52 of the layer, and laser scanning speed.

[0053] In practicing the present invention, the stereolithographyapparatus 10 preferably comprises a commercially available STL systemwhich is modified by the invention to enable smoothing of vertical sides54 of STL-formed structures 40. For example and not by way oflimitation, the SLA-250/50HR, SLA-5000 and SLA-7000 stereolithographysystems, each offered by 3D Systems, Inc, of Valencia, Calif. aresuitable for modification. Liquid photopolymers 16 believed to besuitable for use in practicing the present invention include Cibatool SL5170 and SL 5210 resins for the SLA-250/50HR system, Cibatool SL 5530resin for the SLA-5000 system, and Cibatool SL 7510 resin for the 7000system. All of these resins are available from Ciba Specialty ChemicalsCorporation. By way of example and not limitation, the layer thicknessof material 16 to be formed, for purposes of the invention, may be onthe order of about 0.0001 to about 0.030 inch, and more preferably, fromabout 0.001 to about 0.020 inch, with a high degree of uniformity over afield on a surface 30 of a platform 20. It should be noted that layers50 having differing thicknesses 52 may be used to construct a structure40, so as to form a structure 40 of a precise, intended total height 72or to provide different material thicknesses 52 for different portionsof the structure 40.

[0054] The size of the laser beam “spot” 74 impinging on the surface ofliquid material 16 to cure same may generally be on the order of 0.002inch to 0.008 inch, using presently available STL equipment. Resolutionis preferably about ±0.0003 inch in the X-Y plane (parallel to surface30) over at least a 0.5 inch×0.25 inch field from a center point,permitting a high resolution scan effectively across a 1.0 inch×0.5 incharea. Of course, it is desirable to have substantially this high aresolution across the entirety of surface 30 of platform 20 to bescanned by laser beam 28. This area may be termed the “field ofexposure”, such area being substantially coextensive with the visionfield of a machine vision system employed in the apparatus of theinvention as explained in more detail below. The longer and moreeffectively vertical the path of laser beam 26, 28, the greater theachievable resolution.

[0055] Referring again to FIG. 1 of the drawings, it should be notedthat apparatus 10 of the present invention includes a camera 76 which isin communication with computer 12 and preferably located, as shown, inclose proximity to optics and scan controller, i.e., reflectiveapparatus 24 located above surface 30 of platform 20. Camera 76 may beany one of a number of commercially available cameras, such ascapacitive-coupled discharge (CCD) cameras available from a number ofvendors. Suitable circuitry as required for adapting the output ofcamera 76 for use by computer 12 may be incorporated in a board 82installed in computer 12, which is programmed as known in the art torespond to images generated by camera 76 and processed by board 82.Camera 76 and board 82 may together comprise a so-called “machine visionsystem”, and specifically, a “pattern recognition system” (PRS),operation of which will be described briefly below for a betterunderstanding of the present invention. Alternately, a self-containedmachine vision system available from a commercial vendor of suchequipment may be employed. For example, and without limitation, suchsystems are available from Cognex Corporation of Natick, Mass. Forexample, the apparatus of the Cognex BGA Inspection Package™ or the SMDPlacement Guidance Package™ may be adapted to the present invention,although it is believed that the MVS-8000™ product family and theCheckpoint^(R) product line, the latter employed in combination withCognex PatMax™ software, may be especially suitable for use in thepresent invention.

[0056] It is noted that a variety of machine vision systems are inexistence, examples of which and their structures and uses aredescribed, without limitation, in U.S. Pat. Nos. 4,526,646; 4,543,659;4,736,437; 4,899,921; 5,059,559; 5,113,565; 5,145,099; 5,238,174;5,463,227; 5,288,698; 5,471,310; 5,506,684; 5,516,023; 5,516,026; and5,644,245. The disclosure of each of the immediately foregoingreferences is hereby incorporated by this reference.

[0057] In order to facilitate practice of the present invention withapparatus 10, a data file representative of the size, configuration,thickness and surface topography of a pre-existing object 44, forexample, a particular type and design of semiconductor die to bepackaged, is placed in the memory 34 of computer 12. If the pre-existingobject 44, i.e., die, is to be packaged with a leadframe, datarepresentative of the die with attached and electrically connectedleadframe is provided. If packaging material in the form of theaforementioned photopolymer material 16 is to be applied only to anupper surface 86 (or portions thereof excluding active surfacestructures 92) of a die 44 to form upper package surface 88, or to theupper surface 86 and portions or all of the side surfaces 84 of a die, alarge plurality of such dice 44 may be placed on surface 30 of platform20 for packaging, as depicted in FIGS. 2 and 3. If package sides 54 areto be formed, it is desirable that the surface 30 of platform 20comprise, or be coated or covered with, a material from which the atleast partially cured material 16 defining the lowermost layers of thepackage side wall 54 may be easily released to prevent damage to thepackaging. Alternatively, a solvent may be employed to release thepackage side walls 54 from platform 20 after packaging is completed.Such release and solvent materials are known in the art. See, forexample, U.S. Pat. No. 5,447,822 referenced above and previouslyincorporated herein by reference.

[0058] Following mounting of the dice 44 on platform 20, camera 76 isthen activated to locate the position and orientation of each die 44 tobe packaged by scanning platform 20 and comparing the features of thedice 44 with those in the data file residing in memory 34, thelocational and orientational data for each die 44 then also being storedin memory. It should be noted that the data file representing the designsize, shape and topography for the dice 44 may be used at this junctureto detect physically defective or damaged dice 44 prior to packaging andto automatically delete such dice 44 from the packaging operation. Itshould also be noted that data files for more than one type (size,thickness, configuration, surface topography) of die 44 may be placed incomputer memory 34 and the computer 12 programmed to recognize not onlydie locations and orientations, but which type of die 44 is at eachlocation so that material 16 may be cured by laser beam 28 in thecorrect pattern and to the height required to define package side walls54 and to provide a package top surface 88 at the correct level and ofthe correct size and shape over each die 44.

[0059] Continuing with reference to FIGS. 1, 1A and 1B of the drawings,dice 44 on platform 20 may then be partially submerged below the surfacelevel 18 of liquid material 16 to a depth the same as, or greater than,the thickness of a first layer of material 16 to be at least partiallycured to a semisolid state to form the lowest layer 50A of a packageside wall 54 about each of dice 44, and then raised to a depth equal tothe layer thickness, the surface of liquid material 16 being allowed tosettle. The material 16 selected for use in packaging dice 44 may be oneof the above-referenced resins from Ciba Specialty Chemical Companywhich exhibits a desirable dielectric constant, is of sufficient(semiconductor grade) purity, and which is of sufficiently similarcoefficient of thermal expansion (CTE) so that the package structure,i.e., structure 40 and the die 44, itself is not stressed during thermalcycling in testing and subsequent normal operation.

[0060] Laser 22 is then activated and scanned to direct beam 28, undercontrol of computer 12, about the periphery of each die 44 to effect theaforementioned partial cure of material 16 to form a first layer 50A.The platform 20 is then lowered into reservoir 14 and raised to anotherside wall layer thickness-equaling depth increment 52 and the laser 22activated to add another side wall layer 50B. This sequence continues,layer 50 by layer 50, until the package side walls 54 are built up aboutdice 44. A final layer or layers 50 may be applied over a portion or theentirety of the upper surface 86 of dice 44, forming an upper packagesurface 88 thereon. The layer thicknesses 52 may be controlled todiffer, depending upon the thickness required for the top of thepackage. For example, a greater total thickness of material 16 may berequired to cover a die 44 having wire bonds protruding upwardlytherefrom than if a die 44 is covered before connection to a leadframe.It should also be noted that the total thickness of material 16 over aselected portion of a given die 44 may be altered die by die, againresponsive to output of camera 76 or one or more additional cameras 78or 80, shown in broken lines, detecting the protrusion of unusually highwire bond loops or other features projecting above the active surface ofa given die 44 which should be, but is not, covered by the “design” orpre-programmed thickness of material 16 disposed over and at leastpartially cured on upper die surface 86. In any case, laser 22 is againactivated to at least partially cure material 16 residing over each die44 to form a package top 94 of one or more layers 50, top 94 beingsubstantially contiguous with package side walls 54. Laser beam 28 iscontrolled as desired to avoid certain surface features on dice 44, suchas bond pads, which are intended to be exposed for connection tohigher-level packaging by wire bonding, tape automated bonding (TAB)using flex circuits, or the use of projecting conductive connectors suchas solder bumps in a “flip-chip” configuration. It should also be notedthat the package top 94 may be formed within an outer boundary definedby side walls 54 extending above upper (active) surface 86 and forming adam thereabout. In this instance, the platform 20 may be submerged sothat material 16 enters the area within the dam, raised above surfacelevel 18, and then laser beam 28 activated and scanned to at leastpartially cure material 16 residing within the dam. Alternatively, a“skin” may be cured by STL over the top surface 86 of the die 44, andliquid polymer 16 entrapped thereby will be subsequently cured in afinal curing step.

[0061] When the final layer 50 _(n) is formed to complete a selectedportion of the structure 40, platform 20 is elevated above surface level18 of liquid material 16 and excess liquid 16 is drained from theSTL-formed structure 40. At this stage, depicted in FIG. 4, the surfaces66 of vertical sides 54 of the at least partially polymerized structure40 may be somewhat nonplanar, having linear, slit-like, externalhorizontal crevices 56 at the interfaces 58 between adjacent layers 50A,50B, 50C and 50D, as well as between layer 50A and platform 20. Ameniscus 60 comprising a quantity of unpolymerized liquid material 16 isretained or trapped within each crevice 56. Where the vertical sides 54meet the die 44, similar interior crevices 96 may occur along the layerinterfaces 58, being filled with unpolymerized photopolymer material 16which is trapped therein.

[0062] Where the initial lack of planarity of the surfaces 66 of thevertical sides 54 may be tolerated, the structure 40 may be washed toremove all unpolymerized material 16 from the external surfaces 30, 66and 88, including the external crevices 56. The washed structure 40 isshown in FIG. 5, being free of liquid photopolymer 16 in the externalcrevices 56. It should be noted that at this stage, the polymercomprising structure 40 is typically in various stages ofpolymerization, including liquid polymer 16 trapped in internal crevices96. Furthermore, the uppermost layer 50D may comprise a polymerized“skin” which traps unpolymerized or partially polymerized material 16therebelow. Following removal of the structure 40 from the platform 20,a final curing step polymerizes and consolidates the structure 40,including any liquid polymer 16 in the internal crevices 96 or otherwisetrapped within the structure. As exemplified in FIG. 6, empty externalcrevices 56 remain in the side walls 54 of the package, i.e., structure40, following removal from the platform 20 and full cure of thestructure 40. These crevices 56 reduce the effective side wall thickness102 (see FIG. 6) and represent potential weaknesses in the packaging 44.Dust, other debris and moisture may collect in the crevices 56.

[0063] The method and apparatus of the present invention pertain to thestereolithographic polymerization of liquid polymer 16 retained inexternal crevices 56 shadowed by overlying polymerized layers 50 of astructure 40 formed by STL, whereby the side wall surfaces 66 are madesmooth. The smoothing method is enabled by certain modifications toconventional STL apparatus, described infra. The smoothing fills thecrevices 56 to a depth approaching the flat side wall surfaces 66, thusremoving locations where dust and moisture may collect. Smoothing alsoincreases the effective thickness of the protective layer over the die44 for uniform protection, increases resistance of the formed structure40 to damage and environmental contamination by providing a strongerstructure, reduces waste of polymeric material, and is aestheticallypreferred.

[0064] In accordance with the present invention, removal of the platform20 with structure 40 from the liquid photopolymer material 16 isfollowed by draining of excess material 16 therefrom, resulting in theconfiguration depicted in FIG. 4. Residual liquid photopolymer material16 retained in the external crevices 56 and having outer meniscussurfaces 60 is not removed.

[0065] As shown in FIG. 7, the platform 20 on which the object 40 isformed is then reoriented or tilted about a horizontal axis 104 to anacute angle 62 from the horizon. Crevice 56 with liquid meniscus 60 in avertical side 54 is then irradiated by scanning of the laser beam 28 atincidence angle 64 to polymerize the photopolymer. At a minimum, a thin“skin” 98 of partially polymerized material must be formed to containany additional unpolymerized liquid material 16 during subsequentwashing (see FIG. 8). Polymerization of the meniscus liquid 16 resultsin a smooth surface 66 of the vertical side 54. The incident laser beam28 is generally in a vertical orientation, whereby the angle ofincidence 64 between beam 28 and the side surface 66 is equal to angle62, and may be any acute angle between about 5 degrees and about 90degrees. The preferred angle of incidence 64 is between about 10 degreesand about 60 degrees. The desirable angle of incidence 64 to achieve asmooth, fully polymerized surface 66 depends upon the depth of thecrevice 56, the concavity or convexity of the liquid meniscus 60, andthe degree to which the liquid meniscus is shadowed by the overlyinglayer 50.

[0066] As shown in FIG. 8, the side surfaces 66 of structure 40 aresmoothed by the tilted STL formation of “skins” 98 of polymerizedphotopolymer, which typically entrap unpolymerized material 16 ininternal pockets 100 in the side walls 54. The formation of a surfaceskin 98 avoids the use of a high incidence angle 64 such as >60 degreesto reach the innermost portions of the crevices 56 and is achieved withminimum laser energy.

[0067] Following the STL smoothing step, any excess uncured liquidmaterial 16 residing on the surfaces of structure 40 may be manuallyremoved and structure 40 may then be solvent-cleaned and removed fromplatform 20, usually by cutting it free of base supports (not shown).Structure 40 will then be generally subjected to postcuring, as material16 is typically only partially polymerized and exhibits only a portion(typically 40% to 60% ) of its fully cured strength. Postcuring toenhance and accelerate consolidation and complete hardening of structure40 may be effected in another apparatus projecting broad-source UVradiation in a continuous manner over structure 40, and/or by thermalcompletion of the initial, UV-initiated partial cure, and/or by othercuring means.

[0068] In this manner, a structure, i.e., package 40 depicted in FIG. 9,may be formed with smooth, uniform thickness side wall surfaces 66 inminimal time within apparatus 10 and, optionally, a final cure apparatussuch as is well known in the art. In instances where a plurality ofstructures 40 are formed on a relatively large platform 20, it isdesirable that platform actuator 36 have the capability of horizontallytranslating platform 20 above the top of reservoir 14 and while in atilted position to place tilted walls of each structure 40 directlybelow laser beam 28.

[0069] In reference to FIGS. 1 and 10, a preferred embodiment of theapparatus 10 includes a platform 20 which is precisely movable in avertical direction, i.e., along vertical axis Z, and may be tiltedvertically about a horizontal axis or axes 104 (see FIG. 7). In afurther preferred embodiment, platform 20 is also rotatable about anaxis 70 normal to the platform surface 30. Thus, once the platform 20 istilted to a desired incidence angle, it may then be rotated about axis70 to present each side wall 54 in turn to the substantially verticallaser beam 28. The laser beam 28 may be scanned over each longitudinalcrevice 56 of a selected side wall 54 before the platform 20 rotates forpresentation of the next side wall 54. Thus, the number of tiltingoperations and scanning steps may be minimized. Alternately, theplatform 20 may be configured to be first rotated about an axis 70 to adesired position and then tilted about horizontal axis 104. The platformmay then be rotated further about axis 70 as desired to presentadditional side walls 54 to laser beam 28. Alternatively, the platform20 may be righted to a horizontal position after each side wall 54facing in a particular direction is exposed to laser beam 28, rotatedabout axis 70 and then re-tilted.

[0070] It should be recognized that where a plurality of structures 40are formed with varying configurations on a platform 20, an initialtilting step followed by platform rotation and X-Y laser scanning ofeach object is readily accomplished. The (a) tilting, rotation and Zaxis movement of the platform 20 and (b) the scanning operations of thelaser beam 28 are both controlled by a computer program using the datafile already present in the computer memory 34. As previously described,such data may include, for example, at least one parameter such as thesize, configuration, thickness and surface topography of each device tobe packaged, together with the construction details of the package to beformed.

[0071] The STL smoothing step may be performed by merely lifting theplatform 20 above the photopolymer reservoir 14, tilting and scanningwithin apparatus 10. Incident or reflected laser radiation from thesmoothing step may be directed undesirably downwardly into the reservoir14. Thus, as shown in FIG. 1, an opaque member 106 is provided which ismovable across the reservoir 14 to shield liquid photopolymer 16 fromreflected or incident laser radiation during the smoothing step. Theopaque member 106 must be opaque to laser radiation and be resistant todamage therefrom. The opaque member 106 may comprise a somewhat flexibleroll of material or a rigid plate which slides over the reservoir 14,for example. Where the smoothing step is not performed adjacent thereservoir 14, use of the opaque member 106 is not required. Such is thecase if the smoothing step is performed by a separate laser apparatus orthe reservoir 14 is removed from STL apparatus 10 prior to the smoothingstep.

[0072] While the smoothing step has been described above in the contextof using a laser beam 28 scanned along crevices 56, it will beappreciated that a broad beam or flood type radiation source ofappropriate wavelength or wavelength range may be used to expose liquidphotopolymer material 16 in all crevices 56 facing in a given directionat the same time. Moreover, simultaneous exposure of all side walls 54for smoothing might be accomplished either within or outside apparatus10 through the use of a plurality of broad beam or flood type radiationsources surrounding and above platform 20 or other supporting platformand facing downwardly at an appropriate angle. Of course, the severityof the angle required for orientation of the radiation sources woulddepend on the spacing between adjacent structures 40 on platform 20.

[0073] While the foregoing example of the invention shows the packagingof a die 44 on the top 86 and sides 84, the die bottom 108 (see FIG. 9)may also be stereolithographically packaged in a variety ofconfigurations to effect substantially complete sealing of the die. Forexample, the dice 44 may be placed bottom 108 down on an insulativematerial which will comprise the bottom packaging and STL used toconstruct the side walls 54 using the insulative material as a base.Alternatively, a package bottom is first formed on the platform 20 bystereolithography, the die 44 is placed on the package bottom, and theSTL process is continued to erect the side walls 54. In yet anothervariation, where a plurality of dice 44 are secured and electricallyconnected (as by wire bonding, thermocompression bonding, TAB bonding orotherwise as known in the art) to leadframes, the leadframe may beinverted to form the bottom packaging by STL. In still anothervariation, dice 44 may be encapsulated on five sides and then invertedand encapsulated on the sixth, whether it be a “top”, bottom” or “side”.Using machine vision systems, mere inversion of the dice 44 after allbut one side of each is covered and reinitiation of laser scanning maybe used to complete the packages. If certain die features such as bondpads, solder bumps, etc., are to remain unencapsulated, the apparatus 10may be programmed to recognize and avoid such features.

[0074] While the prior discussion describes the invention in terms ofthe packaging of the top 86 and four lateral sides 54 of a semiconductordie 44, the method of the present invention is more broadly applicableto the formation of any structure 40 which is formed from a photopolymer16 in layers 50 by stereolithography. Thus, the structure 40 may standalone or be attached to or adjacent to another object such as a die 44.The small size of semiconductor devices makes the use ofstereolithography particularly advantageous for forming protectivepackaging and other structures 40 on devices and electronic substrates.

[0075] It is notable that the method of the present invention produces asubstantially smooth side wall surface 66 without consuming anyadditional photopolymer in comparison to not using the inventive method,and in fact, may enable (due to enhanced uniformity of wall thickness) adie package wall to be formed with a reduced thickness 102, furtherreducing the already small quantity of polymer material 16 consumed instereolithographic packaging. In addition, the capital equipment expenseof transfer molding processes is eliminated and the inventive method isextremely frugal in its use of dielectric encapsulant material 16, sinceall such material in which cure is not initiated by laser 22 remains ina liquid state in reservoir 14 for use in packaging the next pluralityof dice 44 or other objects. Further, since it is no longer necessary toencapsulate dice 44 with packaging of sufficient wall thickness toaccommodate relatively large dimensional variations such as those whichmay be exhibited by wire bond loop heights, the overall volume ofpackaging material may be smaller in some cases. Also, the packagedimensional tolerances achievable through use of the present inventionare increased in precision in comparison to transfer molded packaging.Moreover, there is no potential for mold damage, mold wear, orrequirement for mold refurbishment. Finally, the extended cure times atelevated temperatures for transfer molded packaging, on the order of,for example, four hours at 175° C., required after removal of batches ofdice from the transfer mold cavities, are eliminated. Post-cure of diepackages formed according to the present invention may be effected withbroad-source UV radiation emanating from, for example, flood lights in achamber through which dice are moved on a conveyor or in large batches.Curing in an oven at, for example, 160° C., is another option whicheffects full curing of liquid polymer 16 in interior crevices 96 andinternal pockets 100.

[0076] Full curing of unpolymerized material 16 retained in the crevices56 without a prior skin formation is not practical because, without aprior solvent wash, droplets and films of liquid material persist on thesurfaces of the structure 40. As a result, full curing without a priorsolvent wash results in substantially nonuniform side wall surfaces 66and upper surface 88. If the structure 40 is first solvent-washedwithout “skinning” of the meniscus photopolymer material 16 in crevices56, the liquid polymer 16 is washed from the crevices 56 so the crevicesremain after full cure.

[0077] It should also be noted that the packaging method of the presentinvention is conducted at substantially ambient temperature, the smallbeam spot size and rapid traverse of laser beam 28 around and over thestructures 44 resulting in negligible thermal stress thereon. Physicalstress on structures 44, i.e., the semiconductor dice and associatedleadframes and bare wires, is also significantly reduced, in thatmaterial 16 is fixed in place and not moved over the dice in a viscous,high-pressure wave front as in transfer molding, followed bycooling-induced stressing of the package. Bond wire sweep is eliminated,as is any tendency to drive particulates in the polymer encapsulantbetween lead fingers and an underlying portion of the active surface ofthe die with consequent damage to the integrity of the active surface.

[0078] While the present invention has been disclosed in terms ofcertain preferred embodiments, those of ordinary skill in the art willrecognize and appreciate that the invention is not so limited.Additions, deletions and modifications to the disclosed embodiments maybe effected without departing from the scope of the invention as claimedherein. Similarly, features from one embodiment may be combined withthose of another while remaining within the scope of the invention.

What is claimed is:
 1. A method of forming a smooth enclosure aboutlateral sides of a rectangular object, comprising: providing arectangular object to be enclosed; forming side walls about the lateralsides of said rectangular object as at least two superimposed,contiguous layers of at least semisolid polymeric material defining atleast one exposed crevice therebetween containing liquid polymericmaterial; and at least partially polymerizing said liquid polymericmaterial in said at least one exposed crevice.
 2. The method of claim 1,wherein said forming said at least two contiguous layers comprises atleast partially polymerizing a photopolymer using a moving beam ofradiation.
 3. The method of claim 2, wherein said liquid polymericmaterial in said at least one exposed crevice comprises a photopolymerand said at least partially polymerizing said liquid polymeric materialin said at least one exposed crevice comprises traversing a beam ofradiation along said at least one exposed crevice.
 4. The method ofclaim 1, wherein said providing said rectangular object comprisesproviding a semiconductor die.
 5. The method of claim 4, furthercomprising covering at least one of a top and a bottom of saidsemiconductor die with said at least semisolid polymeric material. 6.The method of claim 1, further comprising providing a platformconfigured for positioning of said rectangular object thereon.
 7. Themethod of claim 6, further comprising reorienting said platform aboutthe horizontal axis thereof, said platform having said rectangularobject thereon including said sidewalls formed about said lateral sidesthereof.
 8. The method of claim 1, wherein said forming said sidewallsabout said lateral sides of said rectangular object comprises:submerging said rectangular object in a liquid photopolymer material toa first depth over an upper surface of said rectangular object; applyinga beam of polymerization stimulating radiation to selected regions ofsaid liquid photopolymer material over said upper surface of saidrectangular object to at least partially polymerize said liquidphotopolymer material overlying said selected regions and form a firstof said at least two superimposed contiguous layers of at leastsemisolid polymeric material defining at least a portion of a side wall,said first layer extending substantially parallel to said upper surfaceof said rectangular object; submerging said rectangular object in saidliquid photopolymer material to as second depth over said first layer;applying a beam of polymerization stimulating radiation to selectedregions of said liquid photopolymer material over said first layer topartially polymerize said liquid photopolymer material in said selectedregions to form a second layer of at least semisolid photopolymerdefining at least another portion of said side wall, said second layerextending substantially parallel to said first layer, wherein said atleast one crevice is defined between at least one side of said secondlayer and said first layer and said at least one crevice containsunpolymerized liquid photopolymer material.
 9. The method of claim 8,further comprising generating said polymerization stimulating radiationbeam as a laser beam.
 10. The method of claim 8, wherein said orientingsaid rectangular object comprises orienting said rectangular objectabout a horizontal axis thereof.
 11. The method of claim 8, furthercomprising generating said polymerization stimulating radiation asultraviolet radiation.
 12. The method of claim 8, wherein said at leastpartially polymerizing said liquid polymeric material in said at leastone crevice comprises: removing said rectangular object from said liquidphotopolymer material, leaving a quantity of said liquid photopolymermaterial in said at least one crevice; orienting said base to permitaccess to said quantity of said liquid photopolymer material by saidpolymerization stimulating radiation beam; and exposing said quantity ofsaid liquid photopolymer material to polymerization stimulatingradiation to at least partially polymerize at least an outer portion ofsaid quantity of said liquid photopolymer material.
 13. The method ofclaim 12, wherein said exposing said quantity of said liquidphotopolymer material to said polymerization stimulating radiationcomprise traversing said beam of said radiation along said crevice.